Various inbred strains of mice and rats will be immunized with antigens such as sheep erythrocytes or the haptens oxazolone, 2,4-dinitrophenyl and 2,4,6 tri-nitrophenyl by protocols that result in the generation of suppressor cells. Suppressor factors with specific antigen binding capability will be purified from cell cultures and characterized by physical, chemical and immunological techniques. These techniques will answer questions about the size, degree of homogeneity, and antigens binding properties of suppressor factors. Anti-sera will be prepared against purified suppressor molecules and will be used in both in vitro and in vivo experiments designed to investigate the biological role of suppessor factors.